This came up on perl5-porters this week:

The question is, when does the object get destroyed given this code?

if (my $object = Foo->new) {}
print "after if\n";

package Foo;
sub new { my $self = bless {},shift; print "CREATED $self\n"; $self }
sub DESTROY { print "DESTROYED $_[0]\n" }
[download]

Well, contrary to expectation (well, at least mine), the object does not get destroyed until global destruction! As the following output shows:

CREATED Foo=HASH(0xfc65c)
after if
DESTROYED Foo=HASH(0xfc65c)
[download]

instead of the expected:

CREATED Foo=HASH(0xfc65c)
DESTROYED Foo=HASH(0xfc65c)
after if
[download]

It appears that this behaviour is intentional and goes back until at least 5.00504. It is caused by the fact that any lexical inside an if() gets a refcount of 2, rather than of 1.

If this really is a problem for you, then there is the Internals module which allows you to set the refcount of a variable from Perl.

use Internals qw(SetRefCount);
if (my $object = Foo->new) { SetRefCount( $object,1 ) }
print "after if\n";

package Foo;
sub new { my $self = bless {},shift; print "CREATED $self\n"; $self }
sub DESTROY { print "DESTROYED $_[0]\n" }
__END__
CREATED Foo=HASH(0xfc65c)
DESTROYED Foo=HASH(0xfc65c)
after if
Attempt to free unreferenced scalar: SV 0xfc65c.
[download]

Note that the object now is destroyed at the end of the if(), but that we get a warning at global destruction as a bonus.

Of course, there is a much simpler method: just add an extra scope!

{
if (my $object = Foo->new) { }
}
print "after if\n";

package Foo;
sub new { my $self = bless {},shift; print "CREATED $self\n"; $self }
sub DESTROY { print "DESTROYED $_[0]\n" }
__END__
CREATED Foo=HASH(0xfc65c)
DESTROYED Foo=HASH(0xfc65c)
after if
[download]

Hope this maybe helpful for someone someday.

Liz

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